In recent years, in order to achieve high breakdown voltage, low loss, and the like in a semiconductor device, silicon carbide (SiC) has begun to be adopted as a material for the semiconductor device. Silicon carbide is a wide band gap semiconductor having a band gap larger than that of silicon, which has been conventionally widely used as a material for semiconductor devices. Hence, by adopting silicon carbide as a material for a semiconductor device, the semiconductor device can have a high breakdown voltage, reduced on-resistance, and the like. Further, the semiconductor device thus adopting silicon carbide as its material has characteristics less deteriorated even under a high temperature environment than those of a semiconductor device adopting silicon as its material, advantageously.
Because silicon carbide has a very low diffusion factor for impurity, it is difficult to dope it with an impurity by means of a thermal diffusion treatment. Examples of a method of forming an active region in a silicon carbide material includes: a method of performing ion implantation into an epitaxial growth layer; and epitaxial growth method involving addition of impurity by way of a dopant gas.
Silicon carbide is epitaxially grown on a silicon carbide single crystal substrate serving as a seed substrate. In doing so, the silicon carbide single crystal substrate is subjected to a surface treatment such as mechanical polishing or chemical polishing, and then the epitaxial film is grown on the surface. Therefore, the surface treatment may cause polishing marks or roughness on the surface of the silicon carbide single crystal substrate.
Moreover, even in the case where the surface of the silicon carbide single crystal substrate is made flat, foreign matters or carbide, silicide, silicon carbide, and the like grown on or adhered to each member in a reaction chamber of a vapor phase epitaxy apparatus may be adhered to or deposited on the surface of the silicon carbide single crystal substrate when epitaxially growing silicon carbide on the surface thereof. In this case, the epitaxial growth is suppressed from being uniformly performed on the surface of the silicon carbide single crystal substrate, whereby the surface of the obtained silicon carbide semiconductor substrate becomes rough and less flat.
Generally, the problem described above is addressed by providing, as a pretreatment for epitaxial growth, vapor phase etching to the surface of the silicon carbide single crystal substrate using hydrogen.
Hydrogen reacts with carbon and silicon constituting the main surface of the silicon carbide single crystal substrate, and produces hydrocarbon and silicane in the vapor phase. Therefore, by exhausting the hydrocarbon and silicane, it is possible to remove foreign matters of carbide or silicide adhered to or deposited on the main surface of the silicon carbide single crystal substrate and to remove defects formed on the main surface of the silicon carbide single crystal substrate.
However, on this occasion, a clean region on the surface of the silicon carbide single crystal substrate is also etched. Moreover, on this occasion, the carbon atoms and the silicon atoms differ from each other in terms of a rate of reaction with the hydrogen atoms, with the result that the main surface of the silicon carbide single crystal substrate may become rough after the vapor phase etching.
Therefore, when epitaxially growing silicon carbide on the main surface of the silicon carbide single crystal substrate having been through the vapor phase etching using hydrogen to remove the foreign matters or defects on the surface as described above, the obtained surface of the silicon carbide semiconductor substrate becomes rough, thus making it difficult to attain a flat surface.
As a method of producing a silicon carbide single crystal substrate having a highly flat surface, Japanese Patent Laying-Open No. 2005-64383 discloses a method of cleaning a surface of a substrate using a mixed gas of hydrogen gas and propane gas at 1400° C. to 1600° C.